1. Field of the Invention
The present invention relates to an optical amplifier, and more particularly to a gain-clamped semiconductor optical amplifier capable of preventing gain from changing during a period directly before gain saturation by means of a wavelength selective reflector.
2. Description of the Related Art
In an optical communication system, signal light sent from a transmitter suffers from transmission loss when transmitted through an optical transmission line, and arrives at a receiver with a reduced power. When the signal light received at the receiver has a power level less than a predetermined value, then communication may not be properly performed as the signal may have significant reception errors. Accordingly, an optical amplifier is installed between the transmitter and the receiver to amplify the signal light so that the transmission loss can be compensated and the signal light can be transmitted over a long distance with few errors.
Important characteristics in an operation of such an optical amplifier include gain, noise figure, and saturation output power. Erbium Doped Fiber Amplifiers, which have been extensively researched, have high gain, low noise figure, and large saturation output power, and have been widely used in optical networks, whether the networks are part of the communication backbone or are part of a metro network. However, Erbium Doped Fiber Amplifiers have disadvantages in that they are expensive have large sizes and a limited amplification bands.
In contrast, semiconductor optical amplifiers (SOAs) constitute semiconductor media in the form of waveguides and include an optical amplification function according to the gain property of the selected semiconductor. Semiconductor optical amplifiers have a lower cost, smaller size, and have amplification bands which can be relatively and freely adjusted.
However, semiconductor optical amplifiers have a high noise figure and a low intensity of a gain-saturated input signal. Therefore, when signals having high optical output are inputted to semiconductor optical amplifiers, the semiconductor optical amplifiers distort the signals and these distorted signals are transmitted.
In order to solve these problems, a gain-clamped semiconductor optical amplifier having a structure as shown in FIG. 1 has been proposed.
More specifically, FIG. 1 is a prospective view showing a conventional gain-clamped semiconductor optical amplifier 100. The gain-clamped semiconductor optical amplifier 100 includes an n-InP (Indium Phosphate) substrate 101, an InGaAsP (Indium Gallium Arsenic Phosphate) passive waveguide layer 102, an InP spacer 103, a distributed Bragg reflector (DBR) lattice pattern 104, an active layer waveguide 105, a current blocking layer 106, a p-InP clad layer 107, a passivation layer 108, a p-InGaAsP ohmic contact layer 109, an upper electrode 110, and a lower electrode 111.
The semiconductor optical amplifier 100 shown induces oscillation of a Bragg wavelength by means of DBR lattices 104 in both sides, thereby preventing electric charge density in a gain layer from increasing further.
However, in the conventional gain-clamped semiconductor optical amplifier (GC-SOA) shown, a step for forming the DBR lattice pattern is required, and, hence, the process for manufacturing the gain-clamped semiconductor optical amplifier is complicated. However, the wavelength of DBR mode moves toward a short wavelength by changing local refractive index profile due to the local carrier depletion near the gain saturation. The movement of the DBR mode changes the gain value of the amplifier by changing the carrier distribution and carrier density of the amplifier, which induces the gain fluctuation near the saturation region in the gain curve of the typical GC-SOA. Therefore, gain in the intensity of the inputted light changes during a period directly before input saturation. This is more clearly shown in FIG. 2, which plots the gain as a function of the power output.